1. Field Of The Invention
This invention relates to a method and system for sterilizing a metallic surgical instrument with microwave radiation and preventing arcing of the metallic surgical instrument.
2. Discussion Of Related Art
The constant exposure of dental and medical personnel and instruments to saliva and blood in virtually every dental and medical procedure is an ever present hazard and potential contributor to the transmission of infection. A variety of approved instrument sterilization methods are available. However, all methods have drawbacks in relation to surgical instruments or dental handpieces and burs.
The most commonly used method of sterilization, autoclaving, is damaging for almost all high-speed dental handpieces. In a study of dental handpieces claimed to be autoclavable, deterioration of performance was noted in a simulated 3-month period. Furthermore, autoclaving, without pretreatment in an oil emulsion, will destroy the sharp edge of burs.
A second method of sterilization is to apply heat to the instruments. This method has the drawback that temperatures of about 160.degree. C. are required while heat generators to rapidly produce such temperatures are not commonly available. The method has further disadvantages in that the rubber and plastic washers and bushings within the dental handpieces could potentially be damaged and long exposure times (approximately 1 hour) are needed.
Gas sterilization with an ethylene oxide mixture is acceptable for both handpieces and burs. However, this is impractical because of cost of equipment, long sterilization and aeration times involved, and cost of providing adequate protection for personnel.
Alkaline glutaraldehyde (2%) will sterilize equipment, but it must be used for 10 hours to kill spore-forming organisms or tuberculosis microorganisms and is irritating to tissue. It must also be constantly monitored as it is not effective when it is more than 2 weeks old.
Another method of sterilization of medical instruments is to scrub them in a detergent solution and wipe them with alcohol. However, because of their serrated or rough hand-grip surfaces, most instruments cannot be disinfected in this way. Except when dry heat is available, no practical method exists for sterilization of dental burs that will not quickly dull the cutting edge.
The above-described sterilization methods are not as advantageous as using dry heat to sterilize medical, dental and similar tools and instruments, because dry heat causes the least amount of damage, such as dulling or rusting to the tools. Dry heat sterilization requires the application of temperatures on the order of 130.degree. C. to 170.degree. C. for several minutes to destroy all pathogens including spore formers. Unfortunately, such techniques are slow when the tools are placed in hermetically sealed pouches since the heat transfer rate through a pouch is slow. Faster dry heat technology is available but it entails placing the tools in a perforated holder or open tray and using high velocity hot air to accomplish the sterilizing, after which the tools would be sterile but open to the air and subject to recontamination.
Microwave energy has been thought of numerous times in the past as a means of sterilizing materials including food products. However, when microwaves are used directly and indirectly to sterilize metallic tools and instruments, which are commonly used in the medical and dental professions, several problems are immediately apparent. The fact that the tools are metallic means they will be heated only slightly or not at all by the microwaves, unless they happen to be magnetic which is not common. This means an auxiliary heat source is required and that must be capable of direct interaction with the microwaves in order to produce heat which will, in turn, heat the tools and, thereby, sterilize them.
A second major problem which is common with any attempt to sterilize metallic tools or instruments is the production of arcs or corona discharge. This may occur between two tools in close proximity to each other or at the sharp edges, points or tips of a single instrument. Such an arc will actually melt the metal and destroy the usefulness of the tool.
Several approaches have been proposed to circumvent these two major problems. First, sterilization by the indirect application of microwaves has been disclosed in U.S. Pat. Nos. 5,019,344; 5,019,359 and 5,039,495. In those patents it is disclosed to use microwaves to vaporize a liquid sterilant solution and to expose the instruments to either the vaporized sterilant alone or to both the microwaves and the vaporized sterilant. When using microwaves to sterilize the instruments, the instruments are placed in a shielded and pressurized atmosphere produced by the vaporized sterilant. The pressurized atmosphere prevents arcing and aids in sterilizing the instruments in conjunction with the microwaves.
In another microwave sterilization technique, dental instruments are directly exposed to microwaves within a microwave oven. The instruments may be placed in plastic autoclave bags when exposed. This technique suffers from several shortcomings, such as (1) needing to rotate the objects in a three-dimensional manner within the oven to uniformly heat the instrument; (2) needing to shield the oven from energy not absorbed by the instruments that is reflected back to the oven; and (3) requiring either an absorber of microwaves, such as water, or an absorber of radar waves within the oven to prevent arcing.
One way to deal with this problem is to surround the tools with a microwave impervious but absorbent material which will prevent the microwaves from "seeing" the tools but will become hot by itself and transfer its heat to the tools. Such materials, which are often used to make such consumer microwave cookware as browner and pizza trays, are usually bulky and expensive and do not lend themselves to the manufacture of disposable pouches.
Another approach is to use disposable inexpensive materials commonly known as microwave susceptors which can be formed into flexible or rigid pouches or boxes. These are generally made by thin film deposition of metals such as aluminum or steel upon plastic films such as polyester. This, in turn, is usually bonded to paper or paperboard to provide support when heated with microwaves. Such films when exposed to microwaves may rapidly reach temperatures of 200.degree. C. which would be useful for sterilization, however, they have a severe drawback for the application described--namely that they do not prevent arcing of metal objects. The reason is that they are largely transparent to the microwave energy and, thus, some microwaves will be received by a metal object. Though it is well known that microwaves reflect off metals, they actually penetrate the metal surface by about 1 .mu.m. This slight penetration causes an activation of the surface electrons in a random manner and creates a current moving along the surface. If a potential charge builds up on a material to the level where it exceeds the ability of the air to carry away the charge, an arc will occur. It is well known that the build up of charge is particularly prevalent at sharp edges or points of the metal surface. When metallic tools, such as dental explorers, are placed in a pouch or box made of commonly available susceptor products and then heated in a microwave oven arcing will occur between tools in close proximity or at their sharp tips. A common occurrence will be a large bright flash almost immediately after the magnetron is energized. This may cause the susceptor to ignite and vigorously burn, while the sharp tip of the dental explorer will be melted into a ball making it useless. In addition, this arcing can damage the oven.
The inventors have addressed the problem of arcing in microwave susceptor containers by inventing a container structure that allows for sterilization of metallic objects while preventing arcing. The container structure is disclosed in U.S. patent application Ser. No. 08/319,944, filed Oct. 7, 1994.